FIX: avoid ProseMirror handling paste when unauthorized (#35640)

When `authorizesOneOrMoreExtensions` is `false`, we don't call
`ComposerUpload#setup` to add the paste event listener, which originally
handles the `preventDefault`:


https://github.com/discourse/discourse/blob/d1e1c02fcb0cbc6722280afb6e086cb6a8b53882/frontend/discourse/app/components/composer-editor.gjs#L274-L276


https://github.com/discourse/discourse/blob/d1e1c02fcb0cbc6722280afb6e086cb6a8b53882/frontend/discourse/app/lib/uppy/composer-upload.js#L151-L153


https://github.com/discourse/discourse/blob/d1e1c02fcb0cbc6722280afb6e086cb6a8b53882/frontend/discourse/app/lib/uppy/composer-upload.js#L566-L569

This makes it so ProseMirror handles paste events itself, which leads to
base64 data src images. This PR adds a custom paste handler for
`prosemirror-editor` that prevents default in this case.

frontend/discourse/app/static/prosemirror/components/prosemirror-editor.gjs

@@ -25,6 +25,7 @@ import { EditorView } from "prosemirror-view";
 import { getExtensions } from "discourse/lib/composer/rich-editor-extensions";
 import { bind } from "discourse/lib/decorators";
 import { i18n } from "discourse-i18n";
+import { authorizesOneOrMoreExtensions } from "../../../lib/uploads";
 import { buildCommands, buildCustomState } from "../core/commands";
 import { buildInputRules } from "../core/inputrules";
 import { buildKeymap } from "../core/keymap";
@@ -77,6 +78,7 @@ export default class ProsemirrorEditor extends Component {
   @service site;
   @service siteSettings;
   @service appEvents;
+  @service currentUser;
 
   schema = createSchema(this.extensions, this.args.includeDefault);
   view;
@@ -214,6 +216,19 @@ export default class ProsemirrorEditor extends Component {
           next(() => this.args.focusOut?.());
           return false;
         },
+        paste: (view, event) => {
+          // When !authorizesOneOrMoreExtensions, we don't ComposerUpload#setup,
+          // which is originally responsible for preventDefault.
+          if (
+            event.clipboardData.files &&
+            !authorizesOneOrMoreExtensions(
+              this.currentUser.staff,
+              this.siteSettings
+            )
+          ) {
+            event.preventDefault();
+          }
+        },
         drop: (view, event) => {
           if (
             [...event.dataTransfer.items].some((item) => item.kind === "file")

spec/system/composer/prosemirror_editor_spec.rb

@@ -1136,6 +1136,29 @@ describe "Composer - ProseMirror editor", type: :system do
       expect(rich).to have_css("img[alt='img1'][data-orig-src]", count: 2)
     end
 
+    it "avoids triggering upload when unauthorized" do
+      SiteSetting.authorized_extensions = ""
+
+      valid_png_data_uri =
+        "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mP8/5+hHgAHggJ/PchI7wAAAABJRU5ErkJggg=="
+
+      cdp.allow_clipboard
+
+      open_composer
+
+      html = <<~HTML
+          <img src="#{valid_png_data_uri}" alt="img1" width="100" height="100">
+        HTML
+
+      cdp.copy_paste(html, html: true)
+
+      expect(rich).to have_no_css("img")
+
+      composer.toggle_rich_editor
+
+      expect(composer).to have_value("")
+    end
+
     it "merges text with link marks created from parsing" do
       cdp.allow_clipboard
       open_composer